The present invention relates to a method for starting a piston engine of a hybrid drive in an electric travel state. The present invention additionally relates to a hybrid drive for a motor vehicle for performing the method and a motor vehicle equipped with a hybrid drive of this type.
Hybrid drives are a modern form of drive technology in motor vehicles and allow reduced-emission travel. In a hybrid vehicle having the capability for solely electrical travel, i.e., in a so-called full hybrid vehicle, the piston engine must be able to be started during the electrical travel, if a power demand exceeding the electric motor must be covered. In a parallel hybrid drive having a clutch between the piston engine and the electric motor, the piston engine is started by being push-started by closing the clutch. A constant clutch torque is set to start the piston engine, which must be reliably sufficient to overcome the first top dead center of the piston of the piston engine. The electric motor must maintain the applied clutch torque to be able to avoid collapse of the output speed and thus jerking in the driving behavior. In particular to avoid jerking of this type and to absorb torque spikes during the starting procedure, a slight slip is permitted at a converter lockup clutch which is situated between the electric motor and the output train. However, it is problematic that the starting torque required to push-start the piston engine must be provided by the electric motor and is therefore not available for driving the motor vehicle. An ideal starting procedure of the piston engine is to occur, however, without changing the currently existing travel state in regard to noise, vibration, and acceleration levels.
The present invention is concerned with the problem of specifying a method for starting a piston engine of a hybrid drive in an electric travel state, which particularly allows soft starting of the piston engine.
This problem is solved according to the present invention by way of a particular method for starting a piston engine of a hybrid drive in an electric travel state. The hybrid drive includes at least the piston engine, an electric motor, a transmission, a clutch, which is situated between the piston engine and the electric motor, and a converter lockup clutch. The converter lockup clutch is situated between the electric motor and the transmission and switchable into a slip state, in which it transmits a reduced torque or no torque from the electric motor to the transmission. The method includes the acts or operations of setting a slip at the converter lockup clutch, increasing a speed of the electric motor from an initial speed to a starter speed and setting the speed in such a way that the speed and the torque do not change or only change in a predefined range at an output side of the converter lockup clutch. A torque is rapidly applied, on and off, at the clutch situated between the piston engine and the electric motor in such a way that a torque pulse sufficient to overcome a first top dead center of a piston of the piston engine is transmitted to the piston engine. A start of the piston engine is initiated at or close to overcoming the first top dead center, and a predefined slip, and thus a predefined torque, are set at the clutch in such a way that the electric motor, upon reaching its starting speed, both supports a run-up of the piston engine and transmits a positive torque to the converter lockup clutch. The slip of the converter lockup clutch is reduced enough that the speed and the torque do not change or only change in a predefined range at the output side of the converter lockup clutch. Advantageous embodiments are also claimed.
The present invention is based on the general idea of only applying a brief torque pulse on and off to the clutch to overcome a first top compression dead center of a piston of the piston engine. This pulse is supplied by the component inertia, i.e., the mass inertia, of the output-side components of a friction face of the (interrupting) clutch on the output side and the friction face of the converter lockup clutch on the drive side including a rotor of an electric motor, the electric motor being accelerated to a higher speed at uniform output torque before the starting event. The speed difference between electric motor on one hand and transmission on the other hand is achieved by setting a slip at a converter lockup clutch situated between the electric motor and the transmission, so that the required output torque is available at the output side of the converter, while on the drive side of the converter, the electric motor generates a small additional torque to accelerate the above-mentioned inertial masses. It is thus possible for only a part of the torque required for starting the piston engine to have to be reserved by the electric motor, and in particular the torque pulse being used to start the piston engine. Therefore, the method according to the present invention has the special advantage that the torque required to overcome the first top dead center is primarily applied by a mass inertia of the rotating rotor of the electric motor, and the torque to be applied later by the electric motor after the first ignition of the piston engine is significantly less and thus requires less torque reserve.
In an advantageous refinement of the present invention, the starting procedure is triggered in that a torque demand of the hybrid drive exceeds a threshold value, at which a torque applicable by the electric motor no longer corresponds to the torque demand. It may be necessary to engage the internal combustion engine if a power of the electric motor no longer corresponds to a required power level, which may occur in the event of a weak battery or too high required speeds of the electric motor. If an operating state of this type occurs, it is necessary to start the piston engine to support the electric motor, which then also feeds torque into the drivetrain after the run-up, and thus may support and/or relieve the electric motor. However, uncontrolled starting of the electric motor is to be precluded, because it is undesirable from both ecological and also economic aspects. The introduction of the above-mentioned threshold is therefore a simple and effective possibility for precisely fixing the starting time of the piston engine. The threshold value may be fixed at different levels depending on the design of the piston engine and/or electric motor.
Further important features and advantages of the present invention are apparent from dependent claims, the drawing, and the associated description of the drawing.
The features noted above and explained in the following are usable not only in the particularly specified combination, but also in other combinations or alone, without leaving the scope of the present invention.
A preferred exemplary embodiment of the present invention is illustrated in the drawing and described in greater detail in the following description.